1. Field of the Invention
The invention relates generally to a method and apparatus for determining and executing correct handoff decisions in a wireless communications system. Specifically, the invention involves determining the correct handoff time (decisions) based on both a long term and short term measurement of the signal level.
2. Description of the Related Art
When a wireless station is in "standby" mode waiting to initiate a call or to receive a call, the wireless station locks onto or tunes to a particular control channel (CCH). As a mobile wireless station moves around geographically, the mobile station must periodically change to a different CCH according to certain criteria.
It is important to accurately evaluate a received signal in order to, e.g., correctly detect the need to switch to a new control channel (CCH) at the appropriate coverage point. This helps maintain both the integrity of the frequency plan defined by a wireless operator, as well as the wireless subscriber's service quality. A mobile station utilizes the signal strength of the CCH to help determine when to switch to a new CCH.
Also, the ability of a base station, also called a land station, to correctly measure a received signal strength of a mobile station is a core building block of this system requirement. The land station must provide a precise and consistent measure of the mobile station's signal strength. The land station must react appropriately to changes in the signal, without overreacting to signal fluctuations prevalent in radio signals due to fading conditions. Overreacting to fluctuating signals can have as detrimental an impact to proper frequency containment as not being sufficiently reactive to true handoff conditions.
Several approaches currently exist to assess a received signal strength. Basic categories can be categorized as "short term" and "long term" approaches.
Methods that rely exclusively on short term measurements are subject to error due to the highly fluctuating nature of the signal. These methods rely on periodic sampling of the signal at intervals of several milliseconds or seconds. This practice often contributes to poor decisions due to the long latencies that may exist between measurements and the effects of short fades on the signal.
Methods that utilize continual measurements at relatively frequent intervals and then average them over a `long` period of time provide a more even view of the received signal. This averaging approach or long term method, smooths out the affects of short term fades in the signal. Thus, "long" term methods are less over reactive to fluctuating signals. However, averaging methods are flawed since they may build in a historical bias to the signal strength average.
In mobile wireless systems, methods that average over long periods of time tend to include measurements from different geographic locations. Therefore, the long term average may also contain a spatial historical bias. In other words, using an average at a current location to evaluate a CCH signal at the current location can be biased by measurement samples from other geographic locations taken previously. Thus, the historical bias may trigger action based on events that occurred not only in the past, but also in different geographic locations. This historical bias can occur frequently in an urban environment due to the shadowing effects of buildings.
A more specific example of how shadowing can cause the historical bias to cause an erroneous trigger condition is as follows. As a mobile is subjected to a shadow effect, older, higher strength, measurements are replaced in the average by more current, lower strength, measurements and the average signal strength begins to fall. This is consistent with the characteristics of averaging algorithms being a "trend follower." As the mobile begins to move out of the shadow effect, the actual current signal strength begins to rise but the long term average keeps falling. Thus, the long term average can indicate that action is required (e.g. CCH reselect) when in fact action is not required at the instant the trigger, indicating the need to reselect a CCH, is invoked.
Overreacting to fluctuating signals can have a detrimental impact on a wireless station's battery life and therefore operational time and also increases transaction processing on the switching system. Additionally, overreacting increases the possibility of missing terminations (incoming calls) since the mobile station would be involved in unnecessary reselection processing. Not reacting quickly enough to changing signal conditions can result in service interruption and expensive processing to re-acquire service.
Another shortcoming of the long term averaging method is that the RSSI (Received Signal Strength Indication) reported at the time of a handoff trigger contains the smoothed average of the signal over a relatively long time interval. In a truly falling signal scenario, this long term representation of the signal is typically much stronger than the actual current signal. This impacts the effectiveness of the handoff process because a target cell selection is based on which of the adjacent cells measure a signal stronger than the value reported by the source cell (usually using a hysteresis mechanism added to the source measurement). The effect of the long term signal representation on this process is that the signal strength of the adjacent cells often may not exceed the long term representation of the source signal, even though the adjacent cells would be able to exceed the current signal strength value of the source signal by the hysteresis. As a result, handoff action is delayed, which may cause both a violation of the frequency plan as well as reduced voice quality to the mobile subscriber as the signal continues to degrade.